The central goal of the proposed research is to determine environmental effects on the stability of model flavosemiquinones. Air-stable flavosemiquinones have been observed by electron-spin resonance in certain enzymes (1). But the stability of the flavosemiquinone system varies from enzyme to enzyme. We propose the varied redox behavior should be observed by altering the solvent in model flavin systems. Our research strategy is to reduce model flavin compounds in solvents of low dielectric constant and monitor the extent of change in redox behavior. To achieve the proposed goal, we will direct our efforts to the following specific aims: 1. Determine the rate of reduction of flavin model compounds by chemical and biological reductants in aprotic or essentially aprotic media. 2. Determine the relative thermodynamic stability of model flavoquinones, flavosemiquinones, and reduced flavins in solvent mixtures of low dielectric constant, and compare them with reported parameters obtained in protic and polar aprotic media. 3. Establish the effects of the complexity of the flavin substituent at N-l on the environment of the flavin moiety when dissolved in solvents of low dielectric constant. We will conduct a series of studies that employ cyclic voltammetry and probe the environment of relevant compounds with a combination of ultraviolet, electron spin and nuclear magnetic resonance spectroscopy.